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To test competing database writes through FastAPI, send concurrent requests to the app using separate database sessions or connections, coordinate them at the operation where they contend, then verify both the API responses and the committed database state. For async database code, FastAPI’s documented test pattern uses @pytest.mark.anyio, HTTPX AsyncClient, and ASGITransport. The expected result depends on your database, isolation level, and endpoint’s conflict or retry contract.
Choose the test style that matches the code
For ordinary synchronous tests, FastAPI’s TestClient works with regular test functions. If the test must await async database calls, use an async test instead. FastAPI documents the @pytest.mark.anyio and HTTPX AsyncClient approach in its async testing guide; its testing guide covers the synchronous client.
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An async client alone does not make database writes compete. Each request must use an independent transactional context, and the test must arrange overlap at the database operation that matters. Otherwise, it may only demonstrate that requests can be awaited concurrently—not how separate database sessions handle a conflict.
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Start with the smallest scenario that represents the business rule: two writes to the same row, two requests competing for one unique allocation, or two independent writes that should both persist. Arrange known initial data, coordinate requests near the contested operation, and check the outcome after both requests finish.
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import asyncio
import pytest
from httpx import ASGITransport, AsyncClient
@pytest.mark.anyio
async def test_competing_writes(app, independent_test_database_sessions):
# Arrange known starting data in the test database.
# Ensure each request receives a distinct session or connection.
gate = asyncio.Event()
async def submit(payload):
async with AsyncClient(
transport=ASGITransport(app=app),
base_url="http://test",
) as client:
await gate.wait()
return await client.post("/resource/write", json=payload)
tasks = [asyncio.create_task(submit(payload)) for payload in payloads]
gate.set()
responses = await asyncio.gather(*tasks)
# Assert the endpoint's documented conflict behavior.
# Inspect persisted state through a fresh database session.This is a framework-level outline, not drop-in fixture code. Adapt the synchronization point to the application’s critical section. A barrier or injected test hook just before the contested read or write is more deterministic than an arbitrary sleep, which only assumes a particular timing.
- Arrange a known starting state. Seed the test database with the row or invariant the requests will contend over.
- Provide independent sessions or connections. Configure the app’s session dependency or factory so each request has its own transaction rather than sharing a session or outer transaction.
- Coordinate at the contested operation. Use a barrier or test hook at the relevant read/write boundary, not a delay intended to make overlap likely.
- Run the requests concurrently. Gather all results and assert the response status and payload for each request according to the endpoint contract.
- Verify durable state separately. After the requests finish, query through a fresh session or transaction and check the invariant and final row values.
Set expectations from the database’s transaction behavior
Concurrency control is implemented by the database, not by FastAPI. PostgreSQL describes concurrent sessions, MVCC, locks, and isolation in its Concurrency Control documentation. If the purpose is to validate production-relevant locking or isolation behavior, run the test against the same database family and material configuration used by the application. A different backend may report or resolve conflicts differently.
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For PostgreSQL, the expected result can vary by isolation level. Under READ COMMITTED, each command begins with a fresh snapshot. Under SERIALIZABLE, a conflicting transaction can fail with a serialization error; PostgreSQL says the whole transaction must be retried from the beginning. See the PostgreSQL transaction isolation documentation. Make the test assert the behavior your API promises: successful writes, a conflict response, or a retry that eventually succeeds.
Uncommitted changes are not visible to other database sessions; committed changes become visible together. For that reason, inspect results through a fresh session after both requests complete rather than trusting an ORM object already held in memory. PostgreSQL explains transaction visibility and atomicity in its transaction tutorial.
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Choose scenarios and assertions that match the invariant
| Scenario | What to verify |
|---|---|
| Writes to independent rows | Both requests complete as expected, and both intended rows persist. |
| Writes competing for the same row | The result matches the endpoint’s contract for conflicts, last-write behavior, locking, or optimistic concurrency. |
| Requests enforcing one shared invariant, such as a unique allocation | The invariant remains true; the losing request receives the documented error or conflict response, or succeeds after the designed retry. |
| Serializable transaction conflict | The application retries the whole transaction or returns its documented failure response. |
| Database resources initialized during application lifespan | Startup setup runs before the test requests and cleanup runs afterward. |
A status code alone cannot prove the data is correct. Pair response assertions with checks against committed state—for example, whether only one allocation exists, whether the final row contains the permitted value, or whether both independent records were saved.
Handle application lifespan and async resources
HTTPX’s ASGITransport does not automatically trigger ASGI lifespan events. If the app creates its database engine, connection pool, or other required resources during startup, manage lifespan explicitly in the test. FastAPI’s async testing guide points to LifespanManager from asgi-lifespan.
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Keep async resource creation and disposal consistent with the event loop used by the test. FastAPI also describes a “Task attached to a different loop” failure when async resources are used across loops; its guidance is to instantiate loop-dependent objects within async functions, such as startup handling.
Clean up without removing the concurrency
Rollback and savepoint patterns can make tests repeatable, but they must not collapse concurrent requests into one shared transaction. PostgreSQL savepoints let a transaction roll back work performed after a savepoint while retaining earlier work; see its SAVEPOINT documentation. For concurrent writes, each request still needs its own connection or transaction if the test is intended to represent independent sessions.
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SQLAlchemy’s 1.3 transaction documentation shows a legacy rollback-oriented test arrangement using an external transaction and nested savepoints. Treat that example as version-specific: check the documentation and APIs for the SQLAlchemy version in your project rather than copying it unchanged. In particular, a single external transaction shared by all requests may conceal the independent transaction behavior the test is meant to exercise.
Know what an in-process test can establish
An async test using ASGITransport exercises application behavior through the ASGI interface without sending traffic over a deployed network. It can test request handling, database coordination, and the endpoint’s conflict contract, but it does not measure throughput through a production server, proxy, or network. Use a separate load-testing setup if the question is deployed performance; use this test to verify correctness under the intended database contention.
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